What's with the down-votes? At least give a reason!
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LukeJul 13 '12 at 10:03

I agree, this is a perfectly reasonable question. If you're going to downvote, have the courtesy to leave a comment explaining why.
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Richard Smith-UnnaJul 13 '12 at 13:40

Have not downvoted nor upvoted the question, but I surely would say that the formulation is quite "rough" as it stands. It could, however, be a good question if better rephrased.
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nicoJul 13 '12 at 13:41

1 Answer
1

The reason for their behavior is that exactly the fact that even though some of them do indeed get killed, most of them survive -- what you have said. The deer cross the river because that allows for the better continuation of the species rather than if they had stayed on the original side of the river. You could think of it this way: If there was a crowd of 5000 people crossing a field of grass and aliens were abducting them one by one, the actual chances of them abducting you are rather little.

The deer do not live for the continuation of the species, but otherwise this answer is correct. The genes which control the urge to cross the river proliferate despite a small amount of predation by crocodiles. Presumably this is because there is a great reproductive or food payoff of crossing the river to better breedinbg/feeding grounds.
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Richard Smith-UnnaJul 13 '12 at 13:41